The present invention relates to a sintered camshaft for use in an internal combustion engine and a method for producing the camshaft.
A sintered camshaft is known in which a sintered cam piece is joined to a steel pipe in order to provide a light weight structure with having high wear resistance and capable of sustaining high planar pressure and high load.
In this case, in order to improve initial break-in property and wear resistivity of the cam portion, a phosphate film treatment is effected at relatively low temperature, or nitriding treatment is effected or vaporization treatment at relatively high temperature.
In the above described sintered camshaft, if a large amount of alloy components such as Cr is contained in order to improve the wear resistivity, formation of the phosphate film may be interrupted by the alloy component when performing the phosphate film treatment, and therefore, the desired effect is not attained.
On the other hand, if nitriding treatment or vaporization treatment is effected in the cam shaft production process, two separate bending correction steps must be required. That is, in the sintered camshaft, after the cam piece formed of ferrous sintering compositions is assembled to a steel pipe, the assembly is subjected to sintering and diffusion bonding which is simultaneously carried out between the pipe and the cam piece within a sintering furnace. In this case, since the camshaft assembly is positioned within the high temperature furnace, deformation or bending may occur in the pipe. Therefore, after sintering in the sintering furnace, bending correction must be carried out to provide a linear orientation of the shaft. However, after the nitriding treatment or vaporization treatment at high temperature subsequent to the sintering step and grinding step to the cam piece, the bending is again provided since an internal stress remains in the pipe due to the first bending correction step. Therefore, additional bending correction is required after the grinding step. The second bending correction after the termination of the camshaft production requires high accuracy. And if it is impossible to effect the second bending correction, the finally obtained camshaft provided through the various production steps is deemed to be unacceptable commercial use, so that productivity of the cam shaft becomes remarkably lowered.
In order to remove internal stress due to the first bending correction step, an annealing and subsequent nitriding treatment has been proposed as disclosed in Japanese Patent Application Kokai No. 63-162852. This annealing is carried out for the purpose of removing the internal stress without application of any physical force such as the first bending correction. However, in the case of the sintered camshaft, a surface layer of the cam piece may be peeled off at high load operation if the camshaft is subjected to the nitriding treatment.
Therefore, a camshaft is provided in which a film of iron tetroxide (Fe.sub.3 O.sub.4) is formed by the vaporization treatment. On the other hand, Japanese Patent Application Kokai No. 58-3901, which corresponds to U.S. Pat. No. 4,518,563 discloses a method for forming a film of iron tetroxide on a cam shaft which employs ferrous alloy powders containing relatively large amounts of Cr and Cu. In this method, thin films are sufficiently formed under a reduced pressure condition on a surface of the sintered body and around sintering pores. However, the cam may be easily worn out if an excessive amount of iron tetroxide film is formed.